Laser mirrors, such as interference mirrors used in ring laser gyroscopes, are continually exposed to a high-energy plasma environment that degrades the mirror by reducing the oxide (removing oxygen) and by inducing photochromic losses. Optical devices of this type are formed from a “quarter wave” stack of alternating ¼ λ-thickness layers of a relatively high index of refraction material and a relatively low index of refraction material. These layers of material are often deposited on a dielectric substrate by vacuum deposition processes.
To reduce the degradation effects, metal oxides with a relatively high bonding energy to oxygen such as zirconium oxide (ZrO2) are typically used as the high index of refraction material. A layer of ZrO2 is also often typically used as the top layer of the stack in interference mirrors. ZrO2 has a relatively large heat of formation (about −131,490 gram calories/mole of oxygen), and exhibits a relatively high resistance to degradation in plasma environments. ZrO2 is also compatible with the relatively high temperatures (e.g., 450°-550° C.) to which the mirrors are exposed during other ring laser gyro manufacturing steps. Another approach for minimizing the degradation effects involves coating the top layer of the mirror stack with Al2O3, another relatively large heat of formation material (about −134,693 gram calories/mole of oxygen).
Unfortunately, the ZrO2 tends to form a micro-crystalline structure when deposited. This micro-crystalline structure creates scattering sites, increasing the loss of the mirror. One known approach for preventing the formation of the crystalline structure and producing an amorphous, low-scatter material is to dope the ZrO2 with small amounts of a second material such as SiO2. Doping the ZrO2 with a relatively low amount (e.g., 5-15 weight %) of SiO2 in this manner inhibits the micro-crystalline structure, thereby enabling relatively low loss, low scatter and long life mirrors.
The bonding energy of SiO2 is so different with respect to that of ZrO2, however, that the SiO2 is reduced in the presence of a significant energy source such as a plasma environment. This interaction can lead to photochromic loss increase which can degrade the performance of the mirror. The result is a decrease in the performance of the ring laser gyro or other system in which the optical device is used.
There is, therefore, a continuing need for improved mirrors and other optical devices used in high energy applications. In particular, there is a need for mirrors with relatively low scatter and loss when used in the plasma environments. The mirrors should also be capable of relatively long life. To be commercially viable, any such mirrors must be capable of being efficiently fabricated.